Human beings' and the other animals' hearing is subject to limits of a specific bandwidth, and therefore audibility attenuates quickly outside the range of the specific bandwidth. The prior art discloses that frequency dependent amplification of the original sound to enhance audibility. But, in many cases, especially for old person, frequency response is more important than amplitude since their hearing bandwidth is very much limited. There are little tradition amplification can do to improve the audibility without further damaged the ear by excess sound level. To avoid this problem and improve the audibility, one can shift the inaudible bands into the audible bandwidth, and possibly with limited amplification.
Therefore, it is imperative to provide a method of processing a sound track to thereby combine sounds synchronously and therefore enhance audibility thereof without generating any interference.